Vacuum tube amplifying apparatus



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Patented May .11, 1943 2,318,662 VACUUM TUBE AmLFYING APPARATUS ICharles H. Bidwell, Riverdale, and Paul V. Koos,

Manhasset, N. Y.,

assignors to Bell Telephone laboratories, Incorporated, New York, N. Y.,a corporation of New York Application september 11, 1941, serial No.411,100

7 Claims.

vIhis invention relates to amplifying apparatusoi' the vacuum tube type,and more particularly to a device to minimize the occurrence oftransients in the output of such apparatus during intervals of changingindividual vacuum tubes therein.

Certain multistage amplifying apparatus comprising two vacuum tubes maybe arranged such that the sockets for each stage are wired in parallelbut only one tube is normally used in one socket while the other socketis vacant. When it is desired to test the one tube used, it is thepractice to insert a second tube in the vacant socket and thereafter towithdraw the one tube from its socket. During such tube insertion 'andwithdrawal, it has been found that, when the filaments of individualtubes are hot, transients may be caused to occur in the output of theamplifying apparatus. ratus of the aforementioned type is utilized .in amultichannel carrier telephone system in which at least one channel isarranged i'or voice frequency carrier telegraph operation, it has beenfound that such transientsare apt to cause hit. As is well known, hitsmay occasion the false operation'of the receiving telegraph apparatus tointroduce errors in the telegraph message being received thereby.

Accordingly, the present invention contemplates a device to controlamplifying apparatus embodying vacuum tubes such that the tendency tocause transients in the output thereof during the intervals of changingtubes in minimized. d

The object of the invention is to minimize the occurrence of hits in avoice frequency telegraph circuit operating in one channel of amultichannel telephone carrier system.

A familiar type of negative feedback amplier apparatus embodying asingle vacuum tubein each of one or more stages is disclosed in thepatent of H. S. Black, No. 2,102,671 granted December 21, 1937. Incertain carrier signaling systems it may be desirable, as previouslymen- Y same with one or two vacuum tubes in any one stage. However, ashereinbefore pointed out,

Where amplifying appationed, to arrange one or more stages of this am- Aplifying apparatus such .that two vacuum tube sockets are connected inparallel but only one tube `is normally used in circuitfAs is wellknown, it is necessary from time to time to test the one tube tomaintain a minimum tolerable quality of transmission. This may beaccomplished by inserting a second vacuum tube in the vacant socket ofthe circuit and thereafter removingthe one tube therefrom. Due to thenegative feedback feature of the amplier apparatus, the over-all steadygain is essentially the such tube insertion and withdrawal may tend tocause transients in the output of the amplider apparatus,

In a specific embodiment of the present invention, a pair ofcapacitorsis so connected to the screen grids of both vacuum tubes that oneterminal of each capacitor is applied to ground potential while theother terminal of each capacitor is applied toA both the screen grid ofone tube and the positive terminal of the source of direct potentialutilized to energize the anode-cathode circuits of both tubes. This isaccomplished by inserting two screen grid connector pins of differentlengths into two terminals of each socket so that when' the second tubeis inserted in the l vacant socket, the longer pin is eiectivelyconnected in circuit in advance of the shorter pin;

and when the one tube is withdrawn from its socket, the shorter pin iseffectively disconnected from the circuit in advance of the longer pin.As a result, the charges on both capacitors, due to the anode energizingsource, serve to apply varying amounts of potential to the screen gridsof the respective tubes during the insertion and removal thereof fromcircuit to minimize the occurrence of transients in the output of theamplifier apparatus. I

The invention will be readily understood from the following descriptiontaken together with the accompanying drawing in which:

Fig. 1 is a schematic circuit showing a one stage amplifying apparatusembodying the invention;

Fig. 2 is a fragmentary elevational view of a vacuum tube used in Fig.1, and

Fig. 3 is a plan view taken along the'line y3*-3 in Fig. 2.

Referring to Fig. 1, a source of signals, not shown, is applied to theprimary winding III of input transformer II whose secondary winding I2is applied across the control grid-cathode circuit of a multielementvacuum tube I3 which embodies among other elements a screen grid Il vand an anode I5. The anode-cathode circuit of the tube I3 is impressedon the primary winding 20 of output transformer '2l whose secondarywinding 22 is connected to a suitable load, not shown. A source 23 ofdirect potential ywhose negative terminal is grounded and resistor 24 inseries therewith serve to energizethe anodecathode circuit of the tubeI3. A lead 25 including a capacitor 26 serves to by-pass the waves inthe output winding 2li to ground.- A negative feedback path is providedat 35 in' accordance with the Black patent (e. g. Fig. 69 thereof). Theforegoing describes essentially a one stage amplifier apparatusembodying one vacuum tube and utilizing negative feedback of the typedisclosed in the patent of H. S. Black. Obviously, such apparatus maycomprise a plurality of stages, each embodying at least one vacuum tube,and a feedback connection extending from the last to the first stage.

A multielement vacuum tube embodying a screen grid 3I and an anode 32and otherwise identical with the tube I3 is connected in paralleltherewith by means of leads 33, 34 and 36. This means that the controlgrid-cathode and anode-cathode circuits of the tube 30 are applied tothe respective input winding I2 and output Winding 20 inparallel withthe corresponding circuits of the tube I3. Due to such. parallelconnection, the source 23, negative feedback path 35 and lead 25 arealso effective in the circuit of the tube 30 to accomplish therespective purposes mentioned above in connection with tube I3. It isunderstood that only the tube I3 is effectively connected in circuitduring normal operation and the tube 30 is substituted for the tube I3in the event that the tube I3 is to be Withdrawn for testing orreplacement as the circumstance may warrant. In other Words, theterminals of the sockets, not shown, for both the tubes I3 and 30 arewired for parallel operation as mentioned above but only one tube, saytube I3 for example, is inserted in one socket and therefore iseffectively in circuit While the socket to receive the tube 30 isvacant. It is further understood that one tube 30 is similarly connectedin parallel with each tube I3.

In accordance with the invention as shown in Fig. 1,capacitors 40 and 4Ihave corresponding terminals applied over lead 42 to ground poten tial43. The other terminal of the capacitor is connected by lead 44 toterminal 45 of the socket, not shown, associated with the tube I3 whichsocket also embodies another terminal 41. The latter terminal isconnected via a lead 48 ard resistor 49 to a common point 50 whoseextension will be subsequently pointed out.

The other terminal of the capacitor 4I is extended over lead to terminal6I included in the socket, not shown, identied with the tube 30 whichsocket also embodies another terminal 63. This terminal is connected bya lead 64 and resistor 65 to the common point 5D which is extended overleads 66 and 61 to one end of the resistor 24 whose other end is appliedto the positive terminal of the direct potential source 23.

Referring to Figs. 2 and 3, each of the tubes I3 and 30 embodies amongother pins a short pin 10 and a long pin 1I, both cf which latter twopins are jointed by a strap 12 suitably soldered thereto. A lead 5I andthe strap 12 connect the pins 1Il'and 'II to the screen grids I4 and 3Iof the respective tubes I3 and 30.

In the operation of Figs. 1 and 2, it is first assumed that the tube I3is inserted in the socket therefor while the socket for the tube 30 isvacant, and the feedback amplifier apparatus shown in Fig. 1 issatisfactorily operating in the manner discussed in the patent of H. S.Black, supra. Next, it is assumed that the tube I3 is to be Withdrawnfrom its socket for test purposes. So as not to interrupt transmissionthe vacant socket. During such insertion, the longer pin 1I engages` itsassociated terminal 6I in the socket for the tube 30 before the shorterpin 10 engages its associated terminal 63 in the same s'ocket. When bothpins 1Il and 1I are in engagement with the respective socket terminals63 and 6I, the capacitor 4I commences to charge in a circuit comprisingpositive terminal of the source 23, resistor 24, leads 61 and 66, commonpoint 50, resistor 65, lead 64, socket terminal 63, short pin 1I), strap12, long pin 1I, socket terminal 6I, lead 60, capacitor 4I, lead 42 andground potential 43.

As the charge is Ibeing gradually built up on the capacitor 4I by thesource 23, a gradually increasing amount of potential is beingeffectively applied to the screen grid 3I of the tube 30 over a circuitincluding ground potential 43, lead 42. capacitor 4I, lead 6B, socketterminal 6I, long pin 1I, and lead 5I. This means, assuming the filamentof the tube 30 to be hot, that the gradually increasing potential beingimpressed on the screen grid 3I causes the production of a grad- -uallyincreasing amount of space current in the tube 30. When the capacitor 4Iis fully charged, substantially the full amount of the potential of thesource 23 is impressed on the screen grid 3| of the tube 30 over acircuit including positive terminal of source 23, resistor24, leads 61and 66, common point 50, resistor 65, lead 64, socket terminal 63, bothpins 1II and 1I and strap 12, and lead 5I.

Inasmuch as, for the moment, both the tubes I3 and 30 are operativelyconnected in the amplifier apparatus shown in Fig. 1, it is understoodthat the action of the negative feedback hereinbefore referred to asexplained in the patent of H. S. Black, supra, serves to maintain theover-all steady gain substantially identical for the condition of one ortwo tubes in such apparatus.

Asthe tube I3 is -being removed from its socket, the short pin 10,'Fig.2, is disengaged from the socket terminal 41, Fig. 1, thereby removingthe source 23 from the screen grid I4 of the tube I3 from which source apotential was being impressed on the screen grid I4 over a circuitcomprising positive terminal of the source' 23, resistor 24, leads 61and 66, common point 50, resistor 49, lead 48, socket terminals 41 and45, both pins 10 and 1I, strap 12, and lead 5I. During such withdrawalinterval, the long pin 1I being still in engagement with the socketterminal 45, serves to apply the charge on the capacitor 40 to thescreen grid-I4 of the tube I3 over a circuit including ground potential43, lead 42, Acapacitor 40, lead 44, socket terminal 45, long pin 1I andlead 5I.

It is understood that the capacitor 40 was initially charged during theinterval of the insertion of the tube I3 into its socket in a manner'similar to previously described charging of capacitor 4I lduring theinsertion of the tube 30 into its socket. The initial charging circuitfor the tube I3 comprises positive terminal of the source 23, resistor24, leads 61 and 66, common terminal 50, resistor 49, lead 48, socketterminal i through the amplifier, the tube 30 is inserted in 41, shortpin 1U, strap 12, long pin 1 I, socket terminal 45, lead 44, capacitor40, ,lead 42, and ground potential 43.

As the current of the screen grid I4 of tube I3 gradually discharges thecapacitor 40 during the previously mentioned withdrawal of the tube I3from its socket, a gradually decreasing amount of potential is beingeffectively impressed on the screen grid I4. This causes a gradualasienta current prov ally decreased during withdrawal from itsassoelatedv socket as distinguished from abrupt changes in space currentduring such insertion and withdrawal, the effective current in thesecondary winding 22 of output transformer 2l is maintainedsubstantially at least at such value that will tend to minimize theoccurrence of transients therein. This obviates ts" on a voice frequencytelegraph s ystem operating in a multichannel carrier telephone system.

It is understood that when the tube i3 is inserted into the socketprovided therefor and the tube withdrawn from the socket providedtherefor the capacitors and 4I function in a direction which is thereverse of that described above so as to repeat the same over-allphenomenon; and that the resistorsl 24, 49 and 65 serve to eilect timedelay for charging the capacitors 40 and 4i.

From a switching aspect, it may be considered that either the tube l3 or30 on withdrawal is eifectively a break before break" switch and oninsertion is effectively a make before make switch in so far as thecharging of the capacitors 4l and 4l by the source 23 is concerned. Thisobviates a use of electromagnetic relays, switching keys, or othernecessary eleectrical apparatus in order to connect the source 23 to andfrom normally in circuit, each tube embodying a screen grid, an anodeand two pin .connectors applied to the screen grid, a source 'of directpotential interposed in saidcircuit means to energize said anodes, andmeans actuated by said pin connectors and said source to control themagnitude of ,transientcurrents produced in the output of individualtubes when a second tube is inserted in said circuit means and said onetube is removed therefrom, including a capacitor individual to each tubeand whose charge applies a varying amount .of potential to said screengrids during said insertion and withdrawal of the respective second andone tubes.

5. I n combination in amplifying apparatus, a

j pair of vacuum tubes, each embodying a'screen the capacitors 40 and 4Ito accomplish the func- 1 tion hereinbefore mentioned.

In the above description, it is assumed that the socket terminals and61, and socket terminals 8i and 63 for the respective tubes I3 and 30ar'e substantially in the same plane so that the different lengths ofthe pins 1l and 1i will accomplish the action above described; and it istherefore obvious that the same action may .be Just as expeditiouslyattained by making the pins 10 and 1l the same vlengthbutproportionately changing the contact surfaces of the socket terminals 41and B3 relative to the respective socket terminals 45 and 8l.

W'hat is claimed is:

l. -In` combination in telectrical apparatus, a pair of vacuum tubes,circuit means to connect said -tubes in parallel such that only one tubeis normally in circuit, each tube embodying a screen grid and an anode,va source of direct potential included in said circuit meansto energizesaid anodes, and means responsiveto said source and said tubes duringthe insertion of a second tube in circuit and the withdrawal vof the onetube therefrom to apply varying amounts of potential to the screen gridsof the individual tubes.

2. The combinationv in electrical apparatus according to claim 1 inwhich said potential applying means applies a decreasing amount ofpotential to the screen grid of said one tube during said withdrawalthereof from said circuit means.

3. The combination in electricaly apparatus according to claim l inwhich said -potential applying means applies an increasing amount of Apotential to the screen grid of said second tube during said insertionthereof in said circuit means. I

4. In combination in amplifying apparatus, a

pair of vacuumltubes, circuit means to connect l said tubes in parallelsuch that only one tube is grid, an anode and a pair of pi'n connectorsof diiferent lengths connected to said screen grid,

circuit means to connect said tubes in.fpara1lel such that only one tubeis normally in circuit, a source of direct potentialto energize-saidanodes, andv means responsive to said tubes and said source to apply avarying amount of potential to said screen grids both during theinsertion of. a second tube in said circuit means and during thewithdrawal of said one` tube therefrom, comprising a pair of terminalsand a capacitor individual to each tube and arranged such that oneterminalof each capacitor is connected to ground potential and the otherterminal of each capacitor is connected to one terminal of each of saidpairs of terminals and the other terminal of each of said pairs ofterminals is connected to the positive terminal of said source, saidpair of pin connectors and said pair of terminals individual to eachtube cooperating such that during said insertion of said second tubeboth said pin connectors engage one pair of said pairs of terminals toconnect one capacitor in sequence to the screen grid of said second tubeand said source to apply eifectiveiy a gradually increasing amount ofp0-v tential to the screen grid of said second tube, and during saidwithdrawal of said one tube the shorter pin connector is disengaged fromsaid other terminal of a second pair of said pairs of terminals todisconnect eilectively said source from .the other capacitor and thescreen grid of said one tube a certain time before the longer pinconnector'is disengaged from the one termilnal of said second pair ofterminals to disconnect the other capacitor from the screen grid of'said one tube to apply effectively a gradually decreasing amount ofpotential to the screen grid of said one tube. I 6. 'A wave amplifyingsystem comprising an input and an output, amplifying means interposedbetween said input and output, a path supplying waves from said outputto said input to control the gain of said system, said amplifying meanscomprising atleast a pair of vacuum tubes connected in parallel suchthat only one tube is normally in said system, each tube embodying ascreen grid, an anode and a pair of pin connectors connected to thescreen grid, a source of direct pbtentlal to energize said anodes, andmeans to* apply varying amounts of potential to said screen grids when asecond tube is inserted 'in said sys-- temand when said'one tube isremoved vthere from, comprising a pair of capacitors. end circuitmeans'to connect said capacitors and .said s source to said pinconnectors such that one terminal of each capacitor 1s applied to groundpotential and the other terminal of each capacitor is connected to thepin connectors of an indii vidual tube and thereby to the positiveterminal oi said source and the screen grid of the indi-v vidual tube.

7. The wave amplifying system according to claim 6 in which said two pinconnectors or each. tube have diii'erent lengths, and said potentialapplying means embodies a pair ofother terminals individual to eachtube, a irst terminal of each pair of said other terminals is connectedto said one capacitor terminal and a second terminai of each pair ofsaid other terminals is connected to said source so that said secondtube on said insertion causes the longer pin connector to tor and thescreen grid oi said one tube a certain time interval before the longerpin connector causes the other capacitor to be disengaged 10 from thescreen grid of said one tube.

CHARLES H. BIDWELL. PAUL V. KOOS.

